Add support for daily builds

https://tools.taskcluster.net/hooks/project-servo/daily

.taskcluster.yml

@@ -7,6 +7,10 @@ tasks:
     then:
       $if: 'event.ref in ["refs/heads/auto", "refs/heads/try"]'
       then:
+
+        # NOTE: when updating this consider whether the daily hook needs similar changes:
+        # https://tools.taskcluster.net/hooks/project-servo/daily
+
         taskGroupId: {$eval: as_slugid("decision_task")}
         taskId: {$eval: as_slugid("decision_task")}
         provisionerId: aws-provisioner-v1
@@ -14,7 +18,7 @@ tasks:
         created: {$fromNow: ''}
         deadline: {$fromNow: '1 day'}
         metadata:
-          name: "Servo: decision task"
+          name: "Servo: GitHub push decision task"
           description: ""
           owner: &task_owner ${event.pusher.name}@users.noreply.github.com
           source: &task_source ${event.compare}
@@ -36,11 +40,13 @@ tasks:
             GIT_URL: ${event.repository.clone_url}
             GIT_REF: ${event.ref}
             GIT_SHA: ${event.after}
+            TASK_FOR: ${tasks_for}
             TASK_OWNER: *task_owner
             TASK_SOURCE: *task_source
           command:
             - /bin/bash
             - '--login'
+            - '-e'
             - '-c'
             - >-
               git init repo &&

etc/ci/taskcluster/decision-task.py

@@ -4,14 +4,28 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
-
 import os.path
+import subprocess
 from decisionlib import DecisionTask
 
 
 def main():
-    linux_tidy_unit()
+    task_for = os.environ["TASK_FOR"]
-#    linux_wpt()
+
+    if task_for == "github-push":
+        linux_tidy_unit()
+        #linux_wpt()
+
+    # https://tools.taskcluster.net/hooks/project-servo/daily
+    elif task_for == "daily":
+        # Unlike when reacting to a GitHub event,
+        # the commit hash is not known until we clone the repository.
+        os.environ["GIT_SHA"] = \
+            subprocess.check_output(["git", "rev-parse", "HEAD"]).decode("utf8").strip()
+        with_rust_nightly()
+
+    else:
+        raise ValueError("Unrecognized $TASK_FOR value: %r", task_for)
 
 
 build_artifacts_expiry = "1 week"
@@ -45,6 +59,18 @@ def linux_tidy_unit():
     )
 
 
+def with_rust_nightly():
+    return decision.create_task(
+        task_name="Linux x86_64: with Rust Nightly",
+        script="""
+            echo "nightly" > rust-toolchain
+            ./mach build --dev
+            ./mach test-unit
+        """,
+        **build_kwargs
+    )
+
+
 def linux_wpt():
     release_build_task = linux_release_build()
     total_chunks = 2

etc/ci/taskcluster/mock.py

@@ -15,6 +15,7 @@ Run the decision task with fake Taskcluster APIs, to catch Python errors before
 
 import os
 import sys
+import collections
 from unittest.mock import MagicMock
 
 
@@ -30,7 +31,8 @@ class Index:
         raise TaskclusterRestFailure
 
 
-Queue = stringDate = fromNow = slugId = os.environ = MagicMock()
+Queue = stringDate = fromNow = slugId = MagicMock()
+os.environ = collections.defaultdict(str, TASK_FOR="github-push")
 sys.modules["taskcluster"] = sys.modules[__name__]
 sys.dont_write_bytecode = True
-exec(open("decision-task.py").read())
+exec(open(os.path.join(os.path.dirname(__file__), "decision-task.py")).read())